The service life of an industrial robot is determined by how its mechanical structure and its components are loaded during the movements carried out by the robot during its service life. The mechanics of robots of today is dimensioned such that the robot is to be able to carry out the severest movement pattern conceivable during the whole of its specified service life. Only a small number of robots are run with such movement programs, which are unfavorable for the mechanical structure, and therefore almost all robots of today are mechanically oversized in relation to their specified service life.
A typical industrial robot comprises a number of robot arms, which are rotatable in relation to each other, and a hand provided with a tool attachment. The robot hand is rotatable in two or three degrees of freedom relative to the arm supporting the hand. The robot is provided with a control system which controls the position and the orientation of the robot hand. For each one of the movement axes of the robot, servo equipment comprising a driving motor and a position sensor is provided. The position sensor delivers a signal which is a measure of the angle of rotation of the axis in question. The servo system of each axis is supplied with a reference value for the angle of rotation of the axis, and the driving motor of the axis brings the robot to move in the axis in question until the axis position indicated by the position sensor of the axis coincides with the reference value supplied to the servo system.
To prevent the loads on the mechanical components of the robot, for example bearings, shafts, stays, motor housing and arm attachments, from becoming too high, limits to the maximum permissible torques and speed for each one of the axes of the robots are set. These limits are set prior to delivery of the robot and limit the performance of the robot, that is, the maximum speeds and maximum torques of the axes, during the whole service life of the robot. The limits to the maximum permissible axis torque and axis speed are calculated based on the guaranteed service life of the robot and fatigue diagrams for the mechanical structure. The calculation starts from a worst conceivable case with abnormal movement patterns and with an abnormal number of cycles per unit of time.
The mechanical load on a mechanical component at a certain time depends on several different factors, for example the speed, acceleration, configuration, and load of the robot. This means that if the robot has an advantageous configuration or a small load, the limits set to the maximum permissible axis torque and axis speed may be exceeded without the load of the component becoming too high.